Two-cycle engines generally have exhaust and intake ports in the side walls of a cylinder. The intake ports or inlet valves feed an air and fuel mixture to the cylinder for combustion. The exhaust port opens to an exhaust passage system where the engine's burnt gases are released. Among its several functions, the reciprocation of the piston in the cylinder across each port cyclically seals and opens each port to effect the proper movement of gases through the engine. Since the ports' locations remain fixed within the cylinder, the exhaust port and intake ports are opened and closed at a fixed time and location with respect to the engine's cycle.
The timing of the engine's cycle for opening and closing the exhaust port and intake ports directly affects the entire engine operation, including its horsepower, fuel efficiency, emissions toxicity, and even its ability to sustain continuous operation. In addition, optimal valve timing varies depending upon the engine speed and load. At high engine speeds, keeping the exhaust port open longer will improve engine performance at such higher engine speeds. If such longer duration is permanently fabricated into the cylinder design, however, the engine will perform poorly at low speeds, where a comparatively shorter exhaust-port-open duration produce better performance.
Thus, unless one can vary the otherwise fixed timing of the exhaust port, the engine will only perform optimally at a certain rpm range. A number of attempts to remedy this problem have been made. Commonly these prior art engines provide some mechanism for covering the upper portion of the exhaust port under low speed operation, effectively lowering the top of the exhaust port in the cylinder to reduce the length of time that the port is open during the engine's cycle. Prior art engines also provide a mechanism to vary the height or axial extent of the exhaust port depending upon the engine operation.
In many cases such prior art systems respond inaccurately or relatively slowly to changing engine conditions, such as fast acceleration or deceleration. For instance, some prior art engines vary the axial extent of the exhaust port based upon throttle position. While throttle position may roughly approximate an engine's load, it provides insufficient indication of the engine's current speed. Since the optimal valve position depends upon both engine speed and load, such prior art engines lack accuracy. U.S. Pat. No. 4,399,788 (Bostelmann) teaches the technique of adjusting exhaust valve position based upon exhaust gas pressure measured in the exhaust system. Changes in an engine's speed and power output propagate into the exhaust system, which functions as a pressure vessel. Higher pressure in the exhaust gas system therefore correspond to higher engine speed and power output. Pressure in the exhaust system is therefore sensed by a diaphragm and used to control the position of the exhaust valve.
The assignee of the Bostelmann patent has commercialized such an exhaust valve system that senses gas pressure in the exhaust system and adjusts the exhaust valve position (and, therefore, the timing) based on that pressure. The Bostelmann system, however, is dependent on the exhaust system to function as a pressure vessel, accurately capturing pressure to operate the exhaust valve system. If, however, the exhaust system gets too loose or becomes disconnected, the pressure sensed drops and the exhaust valve system will not function properly.
Accordingly, it would be desirable to have an exhaust valve system that would function independent of the condition of the exhaust system.